Halfords do the complete range of Audi audio cables. Hopefully at your local branch there will be a knowledgable ICE guy to help you. You'll need Quadlock to ISO harness with Audi TT canbus and single Fakra aerial connector adaptor . Might as well buy the extra harness for the steering wheel controls whilst you are there.
The aerial booster from them is no good, I know I tried one, best to ask on the ICE forum on how to improve radio reception. Also you will probably need help with the sub/rear speakers cabling for them to work.